Displacement by sulfur

Oxidation of cyclo(Pro-Pro) with lead tetraacetate afforded the trans-diacetoxy compound (121) in 32% yield. These acetoxy groups could be displaced by sulfur nucleophiles. Thus, with ethanethiol and ZnCl2, it gave the c -3,6-bis(ethylthio) derivative (120) solvolysis by dilute aqueous acid followed by treatment with H2S/ZnCl2 gave the m-dithiol (73CB396), which can be directly oxidized to the episulfide. 

Halogens are displaced by sulfur nucleophiles as in the phthalazine analogue (24) (67T681). In the thiation of the fused uracil (25) the higher reactivity at C-4, for the reasons discussed above, can be used to effect selective thiation in this position (78CPB2497). 

The stereochemistry of the starting material in (b) was discussed in the solu-ltfli Chapter 16. The starting material has a plane of symmetry but is a single diastc -wl centres are inverted during the double displacement by sulfur. You must be... 

Thianthrene-di-, tri-, and tetracarboxylic acids, and a variety of their derivatives, were prepared and polymerized with co-monomers to obtain thianthiene-containing polyimides, aramids and polybenzoxazoles. The multiply substituted thianthrene derivatives were prepared starting with dichloro-substituted benzamide or phthalimide via chlorine displacement by sulfur nucleophiles. The protected carboxyl groups enhanced the displacement reaction to give thianthrene bisamides and imides in good yields. Deprotection with base gave carboxylic acid derivatives. 

Tiichloiomethanesulfenyl chloiide can be reduced to thiophosgene by metals in the presence of acid and by various other reducing agents. The sulfur-bonded chlorine of trichloromethanesulfenyl chloride is most easily displaced by nucleophilic reagents, but under some conditions, the carbon-bound chlorines are also reactive (54). 

Cellulose valerates have been synthesized by conventional methods using valeric anhydride and sulfuric acid catalyst (25,26). Alternatively, the cellulose is activated by soaking in water, which is then displaced by methylene chloride or valeric acid the temperature is maintained at <38° C to minimize degradation. 

In a penicillin synthesis, the carboxyl group was protected as a / -bromophenacyl ester that was cleaved by nucleophilic displacement (PhSK, DMF, 20°, 30 min, 64% yield). Hydrogenolysis of a benzyl ester was difficult (perhaps because of catalyst poisoning by sulfur) basic hydrolysis of methyl or ethyl esters led to attack at the /3-lactam ring. ... 

Cephachlor (35) became accessible when methods for the preparation of C-3 methylenecephalosporins became convenient. The allylic C-3-acetoxyl residue characteristic of the natural cephalosporins is activated toward displacement by a number of oxygen- and sulfur-containing nucleophiles. Molecules such as can therefore be prepared readily. Subsequent reduction with chromium(II) salts leads to the desired C-3... 

The sulfur analogue of the Hauser ortho-substitution rearrangement provides access to an arylacet-ic NSAID. Reaction of the aminobenzophenone 176 with ethyl methylthioacetate and tert-butyl hypochlorite gives the intermediate 178. The reaction probably proceeds by way of formation of the S-chlorinated sulfonium derivative 177 displacement on sulfur will lead to the salt 178. Treatment with triethylamine leads initially to the betaine 179. Electrocyelic rearrangement of that transient intermediate leads, after rearomatization, to the homoanthranilic acid 180. Internal ester-amine interchange leads then to indolone 181 [45]. The thiomethyl group is then removed with Raney niekel. Saponifieation of intermediate 182 affords bromfenac (183) [46J. 

Activity is also retained when oxygen at the 21 position is replaced by sulfur. Preparation of one of these compounds follows a route quite analogous to the foregoing thus, displacement of the mesylate group in the cortisone (56) derivative 57 with the anion from thiopivalic acid affords thioester 58. Reduction of the 11-ketone by means of borohydride affords tixocortol pivalate (59)[13j. 

The finding that the anthelmintic thiazoloimidazole levamisole showed immunoregulatory activity spurred further investigation of this heterocyclic system. Synthesis of a highly modified analogue starts by displacement of bromine in keto ester 149 by sulfur in substituted benzimidazole 148. Cyclization of the product (150), leads initially to the carbinol 151. Removal of the ester group by saponification in base followed by acid-catalyzed dehydration of the carbinol affords the immune regulator tilomisole (152) [28]. 

In the past, various leuco benzoquinone dyes4 were used as mordant dyes but recently they have been displaced by the azo mordant dyes. The reaction of j9-benzoquinone with j9-chloroaniline gives the hydroquinone derivative (5). Compound 5 undergoes oxidation to the corresponding benzoquinone 6. A mixture of hydrosulfite and compound 6 is marketed as a sulfurized vat dye which gives brown and khaki colors. 

CENTRAL RING CONTAINING ONE HETEROATOM Reaction of 2-bromobenzoic acid (1) with chloroSulfonic acid proceeds to afford the sulfonyl chloride 2 treatment with dimethylamine leads to the corresponding sulfonamide (3). Condensation of bromoacid 3 with the anion from thiophenol in the presence of copper powder results in displacement of halogen by sulfur... 

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